Heavy hydrocarbons in the form of petroleum deposits are distributed worldwide and the heavy oil reserves are measured in the hundreds of billions of recoverable barrels. Because of the relatively high viscosity, which can exceed 106 cp, these crude deposits are essentially immobile and cannot be easily recovered by conventional primary and secondary means. The only economically viable means of oil recovery is by the addition of heat to the oil deposit, which significantly decreases the viscosity of the oil by several orders of magnitude and allows the oil to flow from the formation into the producing well.
Steam assigned gravity drainage (SAGD) utilizes two parallel and superposed horizontal wells vertically separated by approximately 5 meters. The process is initiated by circulating steam in both of the wells to heat the heavy oil/bitumen between the wellpair via conduction until mobility is established and gravity drainage can be initiated. During gravity drainage, steam is injected into the top horizontal well and oil and condensate are produced from the lower well.
SAGD is one of the commercial processes that allows for the in-situ recovery of bitumen. SAGD, as an in-situ recovery process, requires steam generation and water treatment, which translates into a large capital investment in surface facilities. Since water-cuts (produced water to oil ratios) are high and natural gas is used to generate steam, the process suffers from high operating costs (OPEX). To compound these issues, the product, heavy oil or bitumen, is sold at a significant discount to WTI, providing a challenging economic environment when companies decide to invest in these operations.
Theses conditions limit the resource that can be developed to those with a reservoir thickness typically greater than 15-20 meters. The primary driver behind this limit is the steam-to-oil ratio, that is, the volume of steam as water, which is required to produce 1 m3 or 1 bbl of oil. During the recovery process, a wellpair must be drilled and spaced such that it has access to sufficient resources to pay out the capital and operating costs. During the SAGD process, heat is transferred to the bitumen/heavy oil, as well as the produced fluids and overburden and underburden. In thinner reservoirs, economics do not allow wells to access sufficient resources, primarily due to high cumulative steam oil ratio (CSOR). A rule of thumb applied by the SAGD industry is SOR of 3.0 to 3.5 as the economic limit. This of course will vary from project to project.
Therefore, a need exits for enhancements in the SAGD process that can minimize the inefficiencies of the process, while maintaining or improving the economic recovery.